A great science populariser, his essay, 'Daedalus or Science and the Future' (1923) predicted many scientific advances, albeit in a rather idealistic way. In 1925, he was involved in an important development of an aspect of enzyme kinetics in biochemistry. Haldane was a keen experimenter, willing to expose himself to danger to obtain data. He suffered crushed vertebrae from one experiment and perforated eardrums from decompression chamber experiments.

In 1928, Haldane, went to the Soviet Union, where he encountered Marxist philosophy of science for the first time and gradually began to author many articles in the Daily Worker. From 1937, Haldane was a full member of the Communist Party and remined devoted until the celebrated Lysenko affair caused him to break with the Party in 1950. Although only a short while before he had considered standing for Parliament as a Communist Party candidate.

His famous `The Causes of Evolution’He is also known for an observation from his essay, `On Being the Right Sizeeferred to as ’, which many have rHaldane's principle. This is that sheer size very often defines what bodily equipment an animal must have. (1932) re-established natural selection as the key mechanism of evolution by explaining it in terms of the mathematical consequences of Mendelian genetics.